1. Field of the Invention
The present invention relates to a medical visible image generating method for generating a visible image of a region to be observed such as abdomen or head including a plurality of kinds of organic tissues such as organs like liver and pancreas, blood vessels, and tumors by using CG (computer graphics) processing and the like according to image data values such as CT values obtained from medical images tomographically taken by using radiographic diagnosis systems such as CT (computed tomography), MRI (magnetic resonance imaging), nuclear medicine, CR (computed radiography), DSA (digital subtraction angiography), and DR (digital radiography) (real time).
2. Description of the Prior Art
There has recently been an increasing demand for tomograpically taking a medical image from a region to be observed such as abdomen by a radiographic medical diagnosis system such as CT, generating a visible image visualizing the region to be observed therefrom, and using this visible image for various purposes such as explanations for patients, academic uses, and surgical planning.
Conventionally known as a technique for generating such a visible image by CG processing is one called volume rendering. First, in a medical visible image generating method using volume rendering, voxels constituting individual spatial coordinate points of a three-dimensional space corresponding to a tomographic region are used for representing a spatial distribution of image data values such as CT values obtained when a region to be observed is tomographically taken. Subsequently, assuming that a line of sight connects a pixel constituting each plane coordinate point of a two-dimensional projection plane and a point of view (projection center), the degree of chromaticity (the degree of color to be rendered) and the degree of opaqueness (the degree of visibility therethrough) are determined for each voxel according to image data values of voxels located on the line of sight. Then, the degrees of chromaticity and opaqueness determined for the individual voxels located on each line of sight are respectively integrated, and thus integrated values are reflected on pixels of a two-dimensional plane located on the lines of sight, so as to generate a two-dimensional visible image of the region to be observed.
By utilizing the fact that image data values obtained by tomography have respective distribution states specific to individual organic tissues, the above-mentioned conventional medical visible image generating method divides the range of image data values into a plurality of segments according to a distribution (histogram) of all the image data values obtained, and sets degrees of chromaticity and opaqueness having their predetermined values within each segment with respect to individual spatial coordinate points having image data values within the segment.
Such a method of setting degrees of chromaticity and opaqueness may be problematic as follows. In the case of organic tissues whose image data values (CT values) differ greatly from each other, as in bone and soft tissues shown in FIG. 5A, they can completely be separated from each other according to the difference in CT values. Therefore, even when predetermined degrees of chromaticity and opaqueness are set in each segment, visualization can be effected such that the difference between these tissues can be seen as shown in FIG. 5B. In the case of organic tissues whose CT values yield a small difference therebetween, as in soft tissue and blood vessel shown in FIG. 6A, they cannot completely be separated from each other according to the difference in CT values. Therefore, as shown in FIG. 6B for example, segments for discriminating the tissues from each other are set at a position where their distributions overlap each other, and predetermined degrees of chromaticity and opaqueness are set for each segment. However, it has been difficult for such a method to effect visualization by which the difference between the tissues can be seen clearly.
Also, in order to speed up the process of arithmetic operations for respectively integrating the degrees of chromaticity and opaqueness of pixels located on each line of sight, the conventional method carries out arithmetic operations while thinning out data concerning a part of voxels. Therefore, visualized images have failed to express delicate feels of color and opaqueness between organic tissues.